Active matrix substrates used for liquid crystal display apparatuses and the like include, for each pixel, a switching element such as a thin-film transistor (hereafter, “TFT”). As such switching elements, TFTs including an amorphous silicon film as an active layer (hereafter, “amorphous silicon TFTs”) and TFTs including a polycrystalline silicon film as an active layer (hereafter, “polycrystalline silicon TFTs”) have been widely used.
In recent years, as the material for the active layers of TFTs, use of oxide semiconductors instead of amorphous silicon and polycrystalline silicon has been proposed. Such TFTs are referred to as “oxide semiconductor TFTs”. Oxide semiconductors have a high mobility than amorphous silicon. Thus, oxide semiconductor TFTs can operate at a higher speed than amorphous silicon TFTs. In addition, oxide semiconductor films are formed by a process simpler than that for polycrystalline silicon films, hence are also applicable to apparatuses required to have such films having a large area. An active matrix substrate including an oxide semiconductor TFT (hereafter, a “TFT substrate”) is disclosed in Patent Literature 1, for example.
When the existing TFT substrate disclosed in Patent Literature 1 is used for a liquid crystal display apparatus, the TFT substrate is provided so as to include storage capacitors as necessary. Such a storage capacitor can be constituted by a storage capacitor electrode (or a storage capacitor wiring), a pixel electrode, and a dielectric layer positioned therebetween. The storage capacitor electrode is formed from, for example, a conductive film from which the gate wiring is also formed.
On the other hand, for example, Patent Literatures 2 and 3 disclose that a decrease in the resistance of an oxide semiconductor film is caused, to thereby collectively form a pixel electrode and a semiconductor layer serving as the active layer of a TFT.